It is widely appreciated that renewable sources of energy are desirable. In particular, in recent years the development of biodiesel has been encouraged in order to find a replacement for fossil fuels in internal combustion engines. Typically, to make biodiesel, vegetable oils are reacted with alcohols (usually methanol), but in any event the majority of research into biodiesel has focussed on combustion of triacylglycerols of vegetable origin.
Animal fat, due to its different chemical composition and production process presents additional challenges when attempted to be used as a source of fuel.
One method of producing biodiesel from animal fat comprises transesterification, which results in fatty acid esters having physical characteristics that are very close to those of diesel fuel. Furthermore, the methyl or ethyl esters of fatty acids can be burned directly in unmodified diesel engines.
Another method of producing renewable diesel from animal fat is to hydrogenate tallow by reacting it with hydrogen at elevated pressure and temperature.
Engines may be run on certain forms of alternative fuel without significant problems; however, the fuel injectors' maintenance schedule must be modified to incorporate more frequent cleaning and nozzle replacement due to deposit formation.
Each of the above methods requires extensive processing of naturally occurring fats in order for them to be usable in conventional engines.
Some research has been conducted into use of neat (i.e. substantially non-chemically altered) fats in internal combustion engines. For example, pre-heating is a simple way to alter properties of viscous fuels like animal fat; however, excessive carbonization may still occur.
Excessive carbonization in the combustion chamber may increase the compression ratio, which may cause rough running or detonation, and may even cause pistons to seize, holes to be burned in the pistons, damage to bearings and potentially broken cranks. Excessive carbonization on the piston and piston ring may result in heat transfer to the cylinder being reduced, resulting in over-heating and/or eventually seizure. Excessive carbonization around the exhaust port and in the exhaust system may cause the engine to choke.